OpenGL Programming Guide

Chapter 1Introduction to OpenGL

OpenGL as a State Machine

OpenGL is a state machine. You put it into various states (or modes) that then remain in effect until you change them. As you've already seen, the current color is a state variable. You can set the current color to white, red, or any other color, and thereafter every object is drawn with that color until you set the current color to something else. The current color is only one of many state variables that OpenGL preserves. Others control such things as the current viewing and projection transformations, line and polygon stipple patterns, polygon drawing modes, pixel-packing conventions, positions and characteristics of lights, and material properties of the objects being drawn. Many state variables refer to modes that are enabled or disabled with the command glEnable() or glDisable().

Each state variable or mode has a default value, and at any point you can query the system for each variable's current value. Typically, you use one of the four following commands to do this: glGetBooleanv(), glGetDoublev(), glGetFloatv(), or glGetIntegerv(). Which of these commands you select depends on what data type you want the answer to be given in. Some state variables have a more specific query command (such as glGetLight*(), glGetError(), or glGetPolygonStipple()). In addition, you can save and later restore the values of a collection of state variables on an attribute stack with the glPushAttrib() and glPopAttrib() commands. Whenever possible, you should use these commands rather than any of the query commands, since they're likely to be more efficient.

The complete list of state variables you can query is found in Appendix B . For each variable, the appendix also lists the glGet*() command that returns the variable's value, the attribute class to which it belongs, and the variable's default value.

OpenGL Programming Guide